While various past applications of rechargeable (secondary) batteries have occasioned need for state of charge indication systems, the art has not realized a convenient and reliable method of yielding such information, except for metal gas batteries in which gas pressure provides a direct indication of state of charge. The presently active field of electric vehicle development portends even more urgent need for ready state of charge indication than past applications. State of charge indication is significant for various reasons in respect of secondary batteries for electric vehicle propulsion. In a principal instance, such indication is the direct equivalent of the customary automobile fuel gage in advising as to the remaining range capability of the vehicle. In further instances, visual indication of state of charge or electrical sensing thereof, is key to avoiding excessive overcharging and in preventing excessive discharge and cell reversal, all of which can substantially shorten the lifetime of what will apparently be a quite expensive battery. With the metal gas battery being considered generally inapplicable as a source of motive power for electric vehicles, the need for suitable charge indication/control system and apparatus is further emphasized.
Presently-known systems for monitoring secondary battery state of charge are discussed in the prior art statement filed pursuant to 37 CFR 1.97 and 1.98 herein.